The present invention relates generally to apparatus and process for producing information indicative of the physical condition of the living test subject, and more particularly relates to improved apparatus and method for measuring a patient's blood pressure. More particularly still, the invention relates to an apparatus and a method for reducing the effects of signal artifact on blood pressure measurements.
The prior art is replete with devices for measuring the blood pressure of a living test subject. Basic to many of these devices and techniques is the inclusion of a pressure applying cuff which, through selective control of the applied pressure, operates to increasingly or decreasingly occlude a blood vessel embraced there within. Various forms of transducers are then employed for sensing phenomena of the blood vessel and/or the blood flow therein as a function of the pressure applied by the cuff. For example, in one well known technique a stethoscope is used to listen to Korotkoff sounds. More complicated methods and apparatus based on the same principle of listening to the Korotkoff sounds replace the mercury manometer with a mechanical or electromechanical pressure gauge and utilize microphone detection of the Korotkoff sounds which are analyzed electrically. In yet another technique, that of oscillometry, the transducer provides a signal having a waveform which pulsates and is in some way proportional to the magnitude and timing of the blood pressure within the blood vessel as a function of the pressure applied by the cuff.
Examples of these latter mentioned oscillometric techniques are found in U.S. Pat. No. 3,903,872 issued Sept. 9, 1975 to William T. Link for Apparatus and Process for Producing Sphygmometric Information, and in the U.S. patent application Ser. No. 578,047, filed May 15, 1975 by Link et al for Apparatus and Process for Determining Systolic Pressure, now U.S. Pat. No. 4,009,709 issued Mar. 1, 1977, both of which are incorporated herein by reference. In the blood pressure measuring techniques of the aforementioned U.S. Patents, a pressure transducer associated with the cuff operates to sense both the pressure applied by the cuff and a small pulsating signal component representative of the pulsations of a blood vessel embraced therein. Employing algorithms described in greater detail in the aforementioned patent and patent application, the signal provided by the cuff transducer, and more particularly the pulsating component thereof, is analyzed and processed by certain control and logic circuitry to obtain indications of the patient's diastolic and systolic blood pressures.
Basic to the accurate determination of blood pressures using the aforementioned and other techniques is the assumption that the signals received from the cuff and subsequently processed are representative substantially only of phenomena associated with the blood pressure within the blood vessel. However, the signal provided by the cuff transducer may occasionally include components whose amplitude and/or frequency constituents are non-representative of the pressure within the blood vessel. These signal components are generally termed artifacts and will be referred to herein as such. These artifacts may arise for a variety of reasons including voluntary and/or involuntary muscle activity on the part of the patient, pressure-related noise occurring in the general environment in which the blood pressure measurements are being taken, bumping and/or jiggling of the air conduits associated with the cuff, etc. In any event, these artifacts may comprise part of the signal received and analytically processed by the particular system and unless identified and someway compensated for, will distort or invalidate the blood pressure measurements ultimately obtained.
For the foregoing reasons, various efforts have been made to identify and/or compensate for the effects of artifact presence in the signal received for analysis. In the instance of the aforementioned Link patents, several successive blood pressure pulses are continually averaged to minimize the effect of an artifact. It will be appreciated that in some instances such modification of data may have little or no effect on the accurate determination of blood pressure values. However in other instances this modification of data may, depending upon its time of occurrence and the particular algorithm used in the analysis, seriously distort the resulting blood pressure determination.
Accordingly it is a principle object of the invention to provide an apparatus and a method for producing information indicative, with increased accuracy, of the physical condition of the living test subject. Included within this object is the provision of a method and apparatus for accurately determining blood pressure.
It is a further object of the invention to provide an improved method and apparatus for measuring blood pressure, which improved method and apparatus is less prone to erroneous blood pressure determinations as a result of artifacts appearing in the sensed signal representative of pressure pulsations within the blood vessel.